The present invention relates to a cooling air system for reducing the thermal loading on components in the turbine high-pressure region of gas turbines.
In turbines in which the gas temperature is above the permissible material temperature of the blading, it is conventional to cool the blades/vanes with air. The cooling air led through the inside of the blade/vane emerges through a plurality of holes/openings, which may be present in the region of the leading edge and trailing edge of the blade/vane aerofoil, with a part of the air moving over the profile surface as a cooling or insulating film. The cooling air necessary is extracted from the high-pressure region of the compressor and, if appropriate, not until downstream of the combustion chamber diffuser. Due to losses, the inlet pressure must be higher than the outlet pressure necessary in the turbine region. It should be noted that compressor air at higher pressure also has a higher temperature, for which reason the cooling effectiveness decreases so that the cooling air quantity must be increased. This has a doubly negative effect. On the one hand, the cooling air which is not available as an oxidizer for the fuel injected into the combustion chamber reduces the engine power directly and, on the other hand, the aerodynamics of the turbine deteriorate with increasing cooling air quantity, so that the power and efficiency are further reduced.
European Published Patent Application No. 0760 051 describes turbine guide vanes with two separate cooling passages within the vane. The leading edge cooling passage, for internal impingement cooling and external film cooling with air outlet against the hot gas stagnation pressure, requires a high cooling air pressure and is therefore fed with compressor air from one of the rearmost compressor stages near the combustion chamber. The trailing edge cooling passage with air outlet in the flow direction of the hot gas can operate with distinctly less cooling air pressure and is, in this region, fed with compressor air from a compressor stage located further upstream and further removed from the combustion chamber. A disadvantage of this arrangement is that the strongly heated vane cooling edge is cooled in a less effective manner by highly compressed, relatively hot cooling air, with the requirement of a high cooling air quantity per unit time or with the result of an unfavorably high material temperature. The rear vane region is more effectively cooled with compressor air of lower temperature and is therefore less critical. Although the system permits the cooling air throughput to be reduced in the rear vane region and therefore permits the aerodynamics to be improved, additional, life-reducing thermal stresses are to be expected due to the xe2x80x9cthermal unbalancexe2x80x9d between the front and rear vane parts. The unchanged high leading edge temperature relative to xe2x80x9cconventionalxe2x80x9d cooled vanes also, de facto, permits no improvement with respect to the life.
German Published Patent Application No. 197 33 148 describes a cooling appliance for the first turbine stage of a gas turbine which operates in accordance with a comparable principle to that described above. The guide vanes of the first stagexe2x80x94and, if appropriate, further stagesxe2x80x94each have an upstream cooling passage and a downstream cooling passage separate from the upstream cooling passage. The two cooling passages are acted upon by airflows which differ with respect to pressure and temperature, more highly compressed and therefore hotter air being employed in the upstream profile region. The cooler airflow is either tapped from a compressor stage further upstream or is obtained directly from the hot, more highly compressed airflow by pressure reduction. The advantages and disadvantages are essentially the same as those described above.
German Published Patent Application No. 695 04 240 describes a cooling method for a gas turbine power installation in which the high-pressure turbine region is cooled by a mixture of cooling air and steam, the steam proportion mass being greatly preponderant. The energy necessary for the generation of the steam is extracted from the gas turbine exhaust gas. A defined pressure drop generated in the cooling airflow is employed as the control parameter for the mass flow of the admixed cooling steam. An appliance for generating the desired pressure drop can be a heat exchanger by which the cooling air temperature is reduced. The actual cooling of the blading takes place xe2x80x9cconventionallyxe2x80x9d, i.e., there is only one cooling passage in each case for a coolant mixture (air/steam) within the blade/vane. The principle is only suitable for stationary installations or installations in fairly large marine vessels.
If xe2x80x9cconventionallyxe2x80x9d cooled guide vanes and rotor blades, i.e., guide vanes and rotor blades cooled by an airflow through the inside of the blade/vane, are considered, the temperature during operation in the region of the leading edge/inlet edge is approximately at the level of that in the region of the trailing edge/outlet edge but is distinctly lower in the central profile region than at the leading edge and trailing edge. The maximum temperature difference is often even larger during accelerations, i.e., in a transient operating condition, because the leading edge and trailing edge heat up even more rapidly than the more voluminous and better cooled central region. Temperature differences induce thermal stresses, which have a negative effect on the component life. The locally extreme material temperatures at the inlet and outlet edges can damage the material grain structure, which likewise leads to a reduction in the life. An increase in the cooling air throughput only provides partial help at the cost of a reduced engine power or a poorer efficiency.
In contrast, it is an object of the present invention to provide a cooling air system for reducing the thermal load on the components in the turbine high-pressure region of gas turbines, which systemxe2x80x94by reducing the temperature differences and the maximum temperatures in the componentxe2x80x94provides a distinct increase in the component life and a noticeable improvement with respect to efficiency and power of the gas turbine overall.
The components affected relate mainly, but not exclusively, to thermally highly loaded blades/vanes. Thus, in addition to guide vanes and/or rotor blades, other parts may also be thermally relieved by this cooling system. The blading is mainly intended to be associated with turbine stages of axial design, i.e., hot gas flows in a mainly axial direction through them.
Each of the blade/vanes may include at least three flow chambers over the profile length within the blade/vane aerofoil. The cooling air guidance from the compressor to the turbine includes at least one heat exchanger, by which the heat content of a partial flow of the cooling air is distinctly reduced. By this arrangement, cooling air is prepared which, despite its high pressure, has a moderate temperature level. The pressure level will also suffice for leading edge outlets from guide vanes and rotor blades near the combustion chamber. This additionally cooled cooling air is fed to the foremost and rearmost flow chambers of the blades/vanes. The at least one flow chamber in the central profile region is acted upon by warmer cooling air, which is supplied directly from the compressor without any deliberate heat exchange. This arrangement achieves, firstly, the effect that the leading and trailing edges of the blades/vanes remain cooler. Because the central blade/vane region is less strongly cooled, the temperature variation over the profile length is leveled out, i.e., the maximum temperature differences occurring are distinctly reduced and also, therefore, the thermally induced component stresses. In consequence, the blades/vanes may be more severely loaded mechanically, for example due to higher rotational speeds/centrifugal forces, or they may have a more thin-walled, filigree and therefore lighter design for the same load. Because of the reduced cooling air average temperature level, the cooling air quantity extracted per unit time may be reduced. The advantages with respect to efficiency and power of the gas turbine overall are apparent. The extra requirements with respect to design and weight for heat exchange and separate cooling air guides are outweighed by the advantages. The cooling air guided through the heat exchanger and the cooling air guided without heat exchange may initially have the same pressure and temperature level (the same or comparable extraction locations) but that different extraction locations with different thermodynamic values are also possible, for example, different compressor stages. The cooling air guidance, however, generally extends from the high-pressure region of the compressor to the high-pressure region of the turbine. In the case of multi-shaft gas turbines, there is a high-pressure compressor and a high-pressure turbine as structurally independent components which are mechanically connected by a separate high-pressure shaft. Such configurations also fall, without limitation, into the field of application of the invention.